JP2009-103063A, JP2010-3004A, and JP2010-223184A disclose apparatus for calculating fuel injection state based on an injected cylinder waveform. The injected cylinder waveform shows pressure change caused by a fuel injection for one cylinder. The injection cylinder waveform can be detected by monitoring fuel pressure supplied to an injector, e.g., a fuel injection valve, by a fuel pressure sensor. The apparatus calculates the fuel injection states based on a behavior of a fuel injection system in which a beginning of pressure drop caused by a fuel injection and a start timing of fuel injection have high level of correlation. For example, the apparatus calculates a start timing of fuel injection based on a beginning of pressure drop detected from the injected cylinder waveform. The apparatus utilizes the calculated fuel injection state to perform a feedback control for an injector. This enables it to control fuel injection state to a desired state with high accuracy.